1. Field of the Invention
This invention relates to amplifiers generally and, more particularly, to differential amplifiers and to differential power amplifiers.
2. Description of the Prior Art
Differential radio frequency (RF) power amplifiers traditionally operate as a class B amplifier. The output power of the amplifier is generally controlled by limiting the input signal to the amplifier. However, the output power from the power amplifier for a given input signal level may be unpredictable, requiring complex circuitry to regulate the output signal power. See U.S. Pat. No. 4,992,753 for one such approach. In portable and/or cellular applications, this may lead to more complex circuitry than desired and an attendant increase in power consumption.
Another problem with conventional differential, tuned output, amplifiers (power or non-power) is the effect of amplifier saturation. When the amplifier saturates, the resonant circuit in the output of the amplifier becomes shunted by a low impedance, drastically lowering the Q of the tuned circuit. This may reduce the efficiency of the amplifier and increase the harmonics generated by the amplifier.
For example, a conventional differential tuned output amplifier (e.g., an IF amplifier or class B power amplifier) has disposed across the differential output thereof a center-tapped inductor as part of the tuned circuit. The power supply for the amplifier is connected to the center tap. When the amplifier saturates, the inductor is shunted by a low impedance, as discussed above. Further, the inductance of the inductor may be different on either side of the center tap, exacerbating the problems with amplifier saturation.
Thus, it is desirable to provide an amplifier design which can maintain the Q of the tuned circuits in the output thereof when saturated. Further, it is desirable to provide an amplifier design which has a well controlled and predictable gain.